1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, and, more particularly, to an ultrasonic diagnostic apparatus which employs a probe formed into a two-dimensional array structure in which small piezoelectric transducers (hereinafter abbreviated to "transducers") are arranged in a lateral direction (scanning direction) and an azimuth direction for transmitting and receiving ultrasonic wave to and from a subject to be diagnosed.
2. Description of Related Art
Hitherto, a variety of ultrasonic probes (hereinafter abbreviated to "probes") of an ultrasonic diagnostic apparatus have been used to meet the object of the diagnosing operation and the scanning performance.
A probe arranged to perform the scanning operation in a specific direction is formed into a one-dimensional array structure in which a plurality of transducers having the same characteristics are arranged in the lateral direction. When a probe of the above-described type is driven in a linear electronic scanning manner, a plurality of the transducers in the probe are used to form a set to be driven simultaneously. By making, at the above-described driving operation, the driving timing to be different between the transducer positioned at the center in the scanning direction and the transducer disposed in the lateral direction from the central tranducer, the phase difference in the generated soundwave of the transducer can be utilized. As a result, ultrasonic beams converged (focused) in the direction of the subject (such as a human body) to be diagnosed, which is perpendicular to the central portion of the transducer group, can be formed. Accordingly, the ultrasonic beam transmitting position can be sequentially electronically shifted in the lateral direction x by driving the overall portion of a plurality of the transducers to be driven while shifting the overall portion by, for example, a distance corresponding to one transducer in the lateral direction x. Thus, the linear scanning can be performed. At the time of receiving the signals, the transducer, which has transmitted the signal, receives the ultrasonic echo signal to convert it into a voltage signal. The voltage signal obtained by the above-described conversion is transmitted to a receiving circuit so as to be reconstructed into an ultrasonic image data before it is displayed on a display such as a TV monitor.
According to a sector method, the driving timing of each of the transducers is changed in such a manner that the ultrasonic beam scanning direction is sequentially changed into a sector shape at each pulse. As a result, the ultrasonic image formed in accordance with the sector method can be displayed on a display after the operations similar to those performed in accordance with the linear method have been performed.
However, the above-described method in which the probe formed into the one-dimensional array structure is used encounters a problem in that the ultrasonic beam is able to scan only the lateral direction x, which is the direction in which the transducers are arranged, that is, only the one-dimensional direction.
Accordingly, a probe formed into a two-dimensional array structure has been developed the scanning direction of which is not limited to the one dimensional direction but is capable of two-dimensionally controlling the scanning direction in the azimuth direction which is perpendicular to the lateral direction as well as the conventionally realized lateral direction x. The probe formed into the two-dimensional array structure is arranged in such a manner that a plurality of the transducers are furthermore arranged in the azimuth direction which is perpendicular to the lateral direction in addition to the transducers arranged in the lateral direction.
In order to drive the probe formed into the two-dimensional array structure to generate the ultrasonic beams to be received in both the lateral direction and the azimuth direction, it might be considered feasible to employ a structure arranged in such a manner that switches are respectively disposed between all of the transducers and the transmitting-receiving portion so as to be sequentially switched to obtain data.
The azimuth directional beam forming can also be performed by using a known acoustic lens.
However, there arise problems in the case where the transmitting-receiving circuits must be respectively provided for a plurality of the transducers which form the two-dimensional array, the overall size of the transmitting-receiving circuit is enlarged excessively, resulting in deteriorated practicality, and the required quantity of calculations is increased and thereby the real time responsibility deteriorates. Furthermore, the structure in which the acoustic lens is employed suffers from a problem in that the directivity for the azimuth direction is previously determined and the focal point for the azimuth direction cannot further be changed as desired.